Field of the Invention
The present invention relates to radio receivers operating in digital radio links and protected with diversity techniques and more precisely concerns a mixed-combination-strategy combiner for receivers operating a high capacity digital radio links and protected with space or angle diversity.
As known, in a radio link connection the bean transmitted can divide into several beams which reach the receiving antenna over different paths for reasons tied to propagation. This can bring fading of the reception signal caused by the interference of the different beams on the receiving antenna. The fadings are frequently a cause of interruption of radio link connections. It is also known that the fading can be flat or dispersive. In the former case all the frequencies of the spectrum in the band of the received signal are equally attenuated. In the latter case the attenuation strikes predominantly only some zones of the spectrum, producing a distortion of the amplitude-frequency response. In reality the two types of fading can occur simultaneously.
It is useful to note that the dispersive fading is evaluated by measurements of dispersion in the band of the reception signal, where by dispersion it is meant the ratio between the maximum and minimum amplitudes of the frequency spectrum in said band. The frequency for which occurs the greatest attenuation is called notch frequency (f.sub.notch).
In an individual receiver affected by fading, whether flat or dispersive, the error rate on the bits received, corresponding to the BER measured at the output of the demodulator, can exceed a certain threshold, so that the receiver is temporarily out of service. Therefore, in order to avoid this serious shortcoming, diversity reception techniques have been known for a long time. In cases of space or angle diversity, which more strictly concern the type of diversity to which the combiner which is the object of the present invention relates, reception equipment has been equipped with two or more receivers connected to a respective antenna or to their own feeders of said antenna. The outgoing signals from the receivers reach the inputs of a combiner which combines them appropriately to generate a single reception signal to be sent to the demodulator.
With the diversity criterion, the performance of the receiving equipment is considerably improved because the combined signal thus obtained has a much lower probability of going off-service than the individual signals received.
To implement the most appropriate combination strategy, it is essential to know theoretically and/or experimentally the transfer function of a transmitting channel affected by fading. For this purpose there have been proposed different mathematical models among which a well known model is the one proposed by Bell laboratories and known as the `three-beam model`. Of the three beams, one is the main one which reaches the receiving antenna directly, while the other two are echoes of the main beam, i.e. beams which reach the receiving antenna by completing multiple paths of different length. The transfer function proposed for the transmission channel comprises parameters whose values are inferable from knowledge of the fading statistics for the particular section considered. Knowing the type of modulation of the transmitted signal, the fading statistics and the total transfer function of the entire transmission system, it is possible to obtain useful information on the signal characteristics at the output of the demodulator, e.g. power, amplitude dispersion, S/N, BER, etc.
This being stated, it is the job of the combiners to act on some parameters of the diversity receivers in order to optimise a preselected characteristic of the combined signal.
Depending on the optimised characteristic, it is possible, generally speaking, to group the combiners in appropriate operating categories.
A first category includes the combiners which compensate for flat fading by carrying out a phasing of the signals present on the two antennas and then adding them together in voltage to obtain the resulting signal. This strategy corresponds to the maximum summing of the resulting amplitude. It should be clarified that by phase is intended an average phase of the received signal, coinciding with the phase of an unmodulated spectral component transmitted in centreband by virtue of the symmetry of the modulation and the equal probability of all the states owing to the randomness of the modulating signal.
The combiners of this category are mainly used in small capacity radio links where, because of the modest band width, conditionings owing to the signal level prevail. It is natural that for large capacity radio links whose performance is more conditioned by the distortion of the amplitude-frequency response, combiners belonging to a second category characterised by tending to compensate for the dispersive fading are more useful.
These combiners perform an adjustment both of the level and the phase for the received signals, so as to obtain amplitude equality and phase opposition of the echoes on the two antennas. Subsequently the adjusted signals are added together to obtain a resulting signal in which the echoes are cancelled. Cancellation of the echoes leads to a flat amplitude-frequency response typically detectable with a spectrum analyser. However, in particular propagation situations, e.g. when the ratios between the main signal level and the level of the echoes have similar values on both antennas, the combiners of the second category are not efficient. Indeed, in the attempt to cancel the echoes, they also cancel a considerable part of the main signal, worsening the signal-to-noise ratio of the combined signal and causing a possible increase in the down time of the equipment.
To overcome the limitations of the previous combiners, combiners belonging to a third and a fourth category have been developed which set out to optimist the overall performance of the system while allowing for flat and dispersive fading.
Combiners of the third category are characterised by the fact that, based on the fading characteristics of the received signals, they switch from one type of operation, tending to compensate for the flat fading, to the type tending to compensate for the dispersive fading. This manner of operating however brings an excessively fast passage between the two operating zones which, as has been amply shown, can cause oscillations not tolerated by the demodulator.
Combiners belonging to the fourth category, which adopt a mixed combination strategy, also try to obviate this shortcoming in the attempt to compensate simultaneously for the flat and dispersive fading.
An example of a diversity reception system adopting said mixed combination strategy is described in Italian patent application no. 22531A/88 for Telettra, inventors Rocco Nobili, Francesco Rasa and Dario Sormani, filed 7 Nov. 1988.
Said known application claims a system for combining at least two received signals under diversity conditions involving at least two receivers, a combiner for signals coming from said receivers and a demodulator, characterised in that there is used a combiner arranged upstream of the demodulator which measures the power and dispersion of the signal combined therein and destined for the demodulator. On said signal it also makes a calculation of BER with intermediate frequency and, on the basis of the BER value thus calculated, acts in an adaptive and dynamic manner on the two signals received depending on the minimization of said BER.
The system is also characterised in that the BER is calculated with the following function based on experimentally obtained results:
BER=10.sup..alpha.P+.beta. +10.gamma..sup.D+.delta.
where P and D are the measured values of the power and dispersion of the combined signal and the parameters .alpha., .beta., .gamma., .delta. are characteristics of the modulation system employed.
The combiner used in the Telettra system is not however without shortcomings, and in particular it does not seem that it eliminates in a definitive manner the shortcoming resulting from generation of oscillations in the combined signal. The cause of this could be the excessive steepness of the exponential functions used to represent the two contributions to the BER, together with the fact of having neglected the `mixed` term, i.e. dependant simultaneously on power and dispersion. Indeed, as better explained below, the function claimed tends to maintain the combiner in the combination state for maximum power even in the presence of a fair dispersion contribution, then passing suddenly into the combination state for the slightest dispersion when the dispersion contribution becomes excessive. This applies evidently also for the contrary behaviour. In practice, it is as though the combiner in question switched from one type of operation to the other, as happens for the third category combiners.
It can thus be inferred that the mixed combination strategy seems to be hardly effectively implemented by the Telettra combiner and the greatest gaps occur unfortunately just in the intermediate zone of the power and dispersion values where said strategy should bring the greatest benefits.
A second possible shortcoming is due to the fact that, in the presence of notch frequencies suddenly variable because of peculiar dispersive fading situations, the system could have trouble converging toward the minimum BER values, or not converge at all. Indeed, in the BER(P,D) function, there is not shown the further dependence thereof on the notch frequency values, to which refer the dispersion values D given on the abscissa of the chart in FIG. 5 of the above mentioned patent. The nature of said dependence is better explained below. The assertions concerning the second shortcoming are supported by the fact that, while in the Telettra application there is expressly stated that the algorithm used for minimization of the BER minimises the function BER(P,D) given above, nothing is said about the type of algorithm used. There are thus no elements for affirming that said algorithm can effectively minimise also a function representing the BER in a more realistic manner, i.e. which also considers its dependence on f.sub.notch.
Accordingly the purpose of the present invention is to overcome the above shortcomings and provide a mixed combination strategy combiner for receivers operating in high capacity digital radio links and protected with space or angle diversity.